In various applications, a planarized structure comprising recesses filled with copper is desired to be formed. In an example of application, it is provided to assemble face-to-face two chips or integrated circuit wafers, each having such a surface planarized so that the chips or wafers can be coupled by direct copper-to-copper bonding. Direct bonding means any bonding performed with no addition of adhesive material. Such a bonding may be performed at ambient temperature or not, and may be assisted or not by an external compression. If no thermo-compression is used, the constraints in terms of evenness of the surfaces to be assembled are significant to obtain a satisfactory bonding.
A structure comprising a surface provided with recesses filled with copper flush with this surface is generally formed by etching the recesses, by filling the recesses with copper, and by planarizing the surface of the structure so that the copper is flush with this surface.
FIGS. 1A to 1D are copies of FIGS. 1A to 1D of French Patent No. FR2947481 (incorporated by reference) which describes successive steps of such a planarizing method, in the case of recesses having widths in the order of 10 μm.
FIG. 1A shows a structure 2 before planarization. The structure comprises a silicon wafer 6 covered with a silicon oxide layer 8 having recesses 11 etched into it. A titanium nitride diffusion barrier layer 9 has then been deposited on silicon oxide layer 8 and a copper layer 12 has been formed on the barrier layer so that copper 12 fills recesses 11. At this step, the upper surface of the copper comprises steps 13.
FIG. 1B shows the structure of FIG. 1A after a first polishing step providing a planar upper surface of copper 12. To achieve this, a chemical-mechanical polishing (CMP) with a planarizing product is carried out. FIG. 1C shows the structure of FIG. 1B after a second polishing step to remove the copper supported by the upper surface of barrier layer 9 and to leave in place copper pads 10 in recesses 11. This second polishing step has been carried out by a chemical-mechanical polishing with a product selectively etching copper over barrier layer 9.
FIG. 1D shows the structure of FIG. 1C after the removal, by a chemical-mechanical polishing, of barrier layer 9 arranged on either side of pads 10.
The method described in relation with FIGS. 1A to 1D appears not to provide the desired results when the dimensions of the recesses and/or of the spaces between these recesses become smaller than 5 μm, and more particularly when these dimensions become smaller than 1 μm.